fix: don't override process variable in DefinePlugin configuration

Following the warning on https://webpack.js.org/plugins/define-plugin/:

> When defining values for process prefer
> `'process.env.NODE_ENV': JSON.stringify('production')` over
> `process: { env: { NODE_ENV: JSON.stringify('production') } }`.
> Using the latter will overwrite the process object which can break
> compatibility with some modules that expect other values on the
> process object to be defined.

Author: drazik

lib/plugins/define.js

@@ -21,9 +21,9 @@ const applyOptionsCallback = require('../utils/apply-options-callback');
  */
 module.exports = function(plugins, webpackConfig) {
     const definePluginOptions = {
-        'process.env': {
-            NODE_ENV: webpackConfig.isProduction() ? '"production"' : '"development"'
-        }
+        'process.env.NODE_ENV': webpackConfig.isProduction()
+            ? '"production"'
+            : '"development"',
     };
 
     plugins.push({